Modern medicine is capable of reconstructing blood vessels, ducts, and perforating lesions of human organs, using special framework-type devices named stents. Use of stents makes it possible to restore the natural function of a defected anatomical structure without having recourse to direct operative interference techniques.
In order to function as effective and reliable endoprostheses, stents must possess a number of specific properties. First and foremost stents must provide unobstructed motion of body fluids through the implanted structure. Such stents must be very flexible and at the same time must be rigid enough to withstand the pressure exerted by the walls of blood vessels and body cavities, with uniform pressure distribution over the entire side surface of the stent. Moreover, the stent construction must be convenient for being transported to the zone of reconstruction and positioned there, as well as must not produce any or at least a minimal injurious effect upon the surrounding tissues in the course of implantation and further functioning.
One state-of-the-art hollow tubular stent is known to have end portions and a surface formed by a plurality of intersecting elastic elements at least part of which are interconnected at the stent ends (U.S. Pat. No. 733,665).
The stent can be of two different diametrical dimensions due to radial deformation of its elastic elements. Before being positioned at the place of reconstruction the stent is deformed so as to minimize its diametrical dimension. Then the stent is placed, in the deformed state, inside a transporting means by arranging it on a special setting bulb or balloon. Once the stent has been transported to the place of reconstruction the setting bulb or balloon is expanded so that the stent diameter is expanded to its maximal value.
It is due to its rigid construction that the stent withstands rather high pressure of the walls of the organ being prosthesized and provides for a uniform distribution of the resultant stresses over the prosthesized surface.
However, the stent in question features but lower elasticity due to a restricted axial deformation, which affects the quality of endoprosthesizing.
Another prior-art stent is known to be in the form of a hollow tubular springlike body made of a material having a shape memory effect (SME). Stents made from such a material are capable of restoring their shape upon a change in the temperature conditions.
The advantages of said stent are determined by the properties of the material it is made from that provides for complete restoration of the stent shape in the zone of reconstruction, as well as a possibility of its convenient withdrawal from the organ being prosthesized upon cooling of the stent. The procedure of the stent positioning is improved, too.
A variety of stent embodiments are possible. In particular, the stent may have a construction disclosed in the aforediscussed invention (U.S. Pat. No. 733,665).
One more stent embodiment presents its construction as a hollow tubular element established by the coils of a wire or the turns of a strip. The construction of such a stent is more elastic since the stent is deformable both radially and axially.
However, with this stent it is not always possible to provide and optimum value of the pitch of spring coils or of strip turns because with too large of a pitch a uniform pressure distribution over the surface being prosthesized is affected, which may result in partial vessel stenosis, whereas in the case of too small of a pitch stent implantation may cause hyperplasia of the intima of the vascular wall in the organ under reconstruction, as well as early thrombotic complications.
Still more stent of the prior art is known to appear as a three-dimensional tubular structure established by a number of interlaced rigid and/or elastic threads arranged in two groups along helical lines directed oppositely to each other. Ends of these helical threads are not connected to one another or to helical portions of other threads, but are arranged loosely at both ends of the tubular structure.
The stent under consideration is elastic and easily deformable, and can be placed in a small-diameter delivery systems; besides, the stent provides for an adequate rigidity and a uniform pressure distribution over the surface being prosthesized.
However, the presence of free ends of threads on the stent end faces can adversely affect the framework properties as a whole. To attain the required rigidity involves increasing the number of threads used, which is undesirable since this may cause intimal hyperplasia and early thrombotic complications. The ends of threads loosely arranged at the ends of the tubular structure can produce an injurious effect upon walls of a blood vessel; in addition, more complex devices are required to deliver the stent to a required location inside a body.
Known in the present state of the art is a stent in the form of a three-dimensional structure formed by interlaced threads arranged in multistart turns of a helical line (RU, A, 1,812,980). The turns form at least two groups featuring opposite senses of the helical line. The thread is made of a material featuring the SME. The ends of threads belonging to different groups are fixedly joined together on the end faces of the three-dimensional structure by, e.g., spot welding or splicing together.
The stent under discussion provides for a required rigidity and a uniform pressure distribution over the surface being prosthesized, as well as possesses elasticity.
It is due to joined together ends of threads on the stent end faces that its placing into a transporting system is simplified. The selected stent material ensures virtually complete restitution of its shape at the place of the prosthesis implantation.
However, an artificial joining of threads results in a local change of the physic-mechanical properties of the stent, which tells negatively on the rigidity and reliability of the stent construction as a whole. Moreover, the presence of artificial joints between the threads on the stent end faces gives one no way of attaining a maximum possible stent transformation which in turn places limitation on a possibility of its placing into a small-diameter delivery systems.